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Volkswagen Jetta, Golf, GTI: 1993-1999, Cabrio 1995-2002 (A3) VW Mk3 timing-belt adjustment and new-style vs olde tensioner


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Last Post: Jun 20, 2012 11:05 PM Last Post By: mk3diy
mk3diy

Posts: 8
Registered: 06/18/12
VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 9:05 AM
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Attachment 180.JPG (108.2 KB)
Attachment 181.jpg (136.2 KB)
Hello folks,

I am about to start a timing-belt job (the 1st one in my life!)

Thanks to Dan Reed, whose post

http://faculty.ccp.edu/faculty/dreed/Campingart/jettatech/t-belt/index.htm

is an inspiring and clear account. It also gives a great virtual tour of the job, complementing the terse illustrations and text of section 15a-9 in the Bentley (A3 Platform) Service Manual

The main issue of course is about adjusting the tensioner.

Trustworthy prophets (incl. Dan Reed) preach for using the old-style tensioner.
However, how about imagining what the VW R&D must have looked like when the new-style tensioner was proposed. Automakers don't just waste money on more expensive, complex and failure-prone items (read "Wheels" by A.Hailey for a more extensive example of what I'm saying).

However, the Bentley gives little info to give the new thermo-tensioner a chance.
And of course a poorly (or wrongly) adjusted thermo-tensioner is bound to fail.

So here are my questions in case someone'd be kind to help:

p.15a-10, Figs.11,12 and related text about installing the camshaft drive belt

Fig.11 :
  • which of the 2 tools is the arrow about?
  • desired tension level ?

Specifically, to the step #5 text:
  • << 'roller' (or rather 'bracket' ?) Torx loose ... >>
  • how would the bracket be aligned in any way if the Torx is loose ?
  • does pre-tensioning (like step #4) need to be applied prior to setting
the 'Torx' degree of freedom?
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 9:46 AM   in response to: mk3diy in response to: mk3diy
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And one more question:

To rotate the engine (to set the crankshaft at TDC), I gather one rotates CW the crankshaft sprocket center-bolt (whose head requires the 3/4" Star shaped Socket)?

http://faculty.ccp.edu/faculty/dreed/Campingart/jettatech/t-belt/Dsc04219.jpg
http://faculty.ccp.edu/faculty/dreed/Campingart/jettatech/t-belt/Dsc04222.jpg

Doing so, is there any danger to over-tighten the center-bolt ?
FKH161

Posts: 7,661
Registered: 12/29/05
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 12:31 PM   in response to: mk3diy in response to: mk3diy
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mk3diy wrote:
Doing so, is there any danger to over-tighten the center-bolt ?

No.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 12:43 PM   in response to: FKH161 in response to: FKH161
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Thanks. Would you also comment on my original post?
FKH161

Posts: 7,661
Registered: 12/29/05
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 12:48 PM   in response to: mk3diy in response to: mk3diy
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I've seen enough of those tensioners fail (ball end breaks off or comes out of socket), that I stick with the old style.

Personal preference I guess, as well as no confusion on how to adjust.
FKH161

Posts: 7,661
Registered: 12/29/05
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 18, 2012 1:18 PM   in response to: mk3diy in response to: mk3diy
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Fig. 11 is old style tensioner... I think they meant to refer to Fig 12.

Bracket swings when torx is loose... causing piston to rise or lower, at which point you align with outer piston as pictured in Fig 12.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 19, 2012 11:07 AM   in response to: FKH161 in response to: FKH161
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Thanks FKH161 for the kind comments.

I'm loking at the brand new piston from
Indeed ball-end & socket do look fragile.
At the same time my (very trustworthy!) parts guy says that this part is meant to last

for the vehicle's lifetime.
Now, a vehicle's lifetime may mean many drive-belt changes and it seems reasonable to
change also this part AT LEAST every OTHER drive-belt change.
Was this the case with the cars where you saw these tensioners fail?

I'll be posting much more on this issue (photos and even some engineering to the rescue!) tonight when I come back from work.
Stay tuned.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 19, 2012 12:02 PM   in response to: FKH161 in response to: FKH161
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Attachment tension3.png (107.2 KB)
Attachment tension2.png (73.7 KB)
Attachment tension1.png (209.9 KB)
Attachment 193.JPG (334.5 KB)
Attachment 191.JPG (92.1 KB)
So I'm looking at the brand new piston by Febi (Bilstein) Germany, and at
2 new Meyle tensioners (new-style and olde: has the larger dia = 83 mm) .

Let's examine some possibilities here:

Hard mechanical Constraints:

A) piston-housing position and orientation
B) tensioner rotation/adjustment axis
C) Belt overall position in space (except close to the tensioner's roller)

As clearly shown by the attached schematic diagrams:

1) There is no reason whatsoever that a properly cold-adjusted tensioner should fail
either at engine start or when engine becomes hot, as the piston is USUALLY 'stuck' in
its most spring-relaxed (i.e. tensioner most tended) position

2) For the same above reason, a properly adjusted tensioner shouldn't fail
due a malfunction of the thermo-piston.

Now the 1M$ question that remains:

What is the proper tensioner cold-adjustment?

Here is what I gather this far:

1) With the torx OUT (i.e. the piston ain't even there yet!)
adjust the tension as with the olde tensioner

2) Mount the piston

3) Set up the piston as per Fig.12 (an my own photo) -
i.e. align the bracket 'pointer' to the piston-spindle, and just tighten the torx

Not that complicated eh?

Now does it make sense to you?
FKH161

Posts: 7,661
Registered: 12/29/05
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 19, 2012 12:17 PM   in response to: mk3diy in response to: mk3diy
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I guess the ones that I have seen fail, were all past their due date (90K+ miles)... and it was always the ball socket that failed.

Shouldn't be an issue then as long as the timing belt & tensioner are replaced before the failure occurs.

Back in the day we used to always use this tensioner, as that is what the dealer listed. Then we saw the failures and heard how everyone else is also going back to the old style... I'm sure there are those who stick with the newer setup.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 19, 2012 7:54 PM   in response to: mk3diy in response to: mk3diy
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Attachment tension4.png (130.1 KB)
Some rectifications and additional comments.

First, about the "To be or not to be" of the new-style tensioner.

We may each have our extreme points, but truth lies somewhere in-between:

Can the olde tensioner still hold the belt when loose?
Probably yes.
As Dan says on his page, this tensioner has both a larger diameter and wider guide-walls.
Ok, but do you want to continue driving this way indefinitely (or until the next belt job)?

Can the new-style tensioner still hold the belt when loose?
Probably not.
Could the thermo-piston(s) fail?
It might.
Will it upon proper installation and replacement frequency?
Probably not.

Note however, that even when properly installed, the olde tensioner will work
suboptimally.
The new-style tensioner has 2 pistons:
  • the inner piston that assures the baseline tension (a bit like the much bigger spring in the new-style alternator.
  • the outer piston that pulls when the temperature goes up

Car parts become lighter and sometimes smarter.
Noone would go back to fighting the olde-style alternator.

So the only issue that remains is that (in the unlikely case) that the new-style tensioner breaks, this may mean a catastrophic failure.
If I was with the VW R&D, I'd have tried to keep the wider roller diameter and guide-walls (to go with what Dan is saying on his page).

Bottom lines:
1) We have an issue of the type "Progress vs bullet-proof"
2) Given that the upper cover doesn't even require tools to go, its occasional
removal might be of definite interest.

Finally, about installing,
the critical difference is that the new-style tensioner is not held in position by the nut, but rather by the inner piston's compressed spring when cold, and also by the outer piston when hot.
eBahn has this procedure better than the printed Bentley manual.

See the attached diagram.
FKH161

Posts: 7,661
Registered: 12/29/05
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 19, 2012 8:57 PM   in response to: mk3diy in response to: mk3diy
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I failed to mention that I have also seen the old style tensioners fail... not due to design, but like the new style, due to age (wear).

A seized old style tensioner, will cause less damage than a broken ball on a new style tensioner (will skip teeth).

A loose belt on the old style is easy to address & should be checked during regular service of the vehicle... lots of shops don't and several that do will actually over tighten them.

Proper installation & servicing is key to getting the best out of any part that is installed.

Didn't VW/Audi update their thermo tensioner (same as yours) to a newer design a while back for the 1.8T's, that didn't include the small ball & socket?

Thanks for your in-depth analysis! I'm sure forum members will find it useful when deciding which way to go.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 20, 2012 10:57 PM   in response to: FKH161 in response to: FKH161
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Very interesting.

I'd be keen to see a picture of the 1.8T tensioner, as indeed mine says it fits (or used to fit ;-) also Audi.
mk3diy

Posts: 8
Registered: 06/18/12
Re: VW Mk3 timing-belt adjustment and new-style vs olde tensioner
Posted: Jun 20, 2012 11:05 PM   in response to: mk3diy in response to: mk3diy
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And here it is:

http://www.audizine.com/gallery/data/500/1910tension_belt.jpg

The (2-rollers !) design looks indeed a lot more robust.

Then - in a couple of years (when I get richer) I'd go for this

http://up.autotitre.com/a3ddb79fef.jpg

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